1. Field of the Invention
This invention is directed to electrical switching apparatus and, more particularly, to an auxiliary contact operator for an electrical switching apparatus, such as a circuit breaker, and, most particularly, to a mechanism-operated cell (MOC) switch operator for a vacuum circuit breaker.
2. Background of Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers. Circuit breakers are generally old and well known in the art. A typical example of a circuit breaker is disclosed in U.S. Pat. No. 5,341,191. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit condition.
Metal-clad switchgear for removable main circuit breakers is well known in the prior art. A typical example of such switchgear may be found in U.S. Pat. No. 4,743,876. Metal enclosed or metal-clad switchgear apparatus are employed with draw-out circuit breakers which may be levered into position or otherwise disposed in a metal-clad switchgear cabinet for operation. Usually a levering-in or racking-in device with associated and appropriate safety interlocks are taught.
It is also known to utilize auxiliary contacts in association with the main interrupter contacts of metal-clad switchgear apparatus. The auxiliary contacts usually perform indicating or control functions and are not meant to carry the main load current, but are meant to open and close in some direct correspondence with the opening or closing of the main circuit interrupter contacts. In some cases the auxiliary contacts are opened and closed in direct conjunction with the opening and closing of the circuit interrupter contacts and in other cases they are not. In cases where the auxiliary contacts are mounted as part of the metal-clad cabinet rather than on the circuit interrupter itself, it is necessary to provide an inter-linkage between the circuit interrupter and the auxiliary contacts when auxiliary contact operation is desirable.
Drawout circuit breakers are well known in the art. A typical example may be found in U.S. Pat. No. 4,565,908. Such circuit breakers include a drawout mechanism which permits an operator to physically disconnect the circuit breaker from a power source and a load which are electrically interconnected with the circuit breaker by terminals at the rear of the circuit breaker.
A mechanism-operated cell (MOC) switch is typically located in a switchgear cell and is operated by a circuit breaker MOC operator mechanism. A typical example may be found in U.S. Pat. No. 4,176,262. The MOC switch is employed to provide extra or redundant contacts for circuit breaker status and status control.
It is well known that modern technology circuit breaker mechanisms operate with different force, travel, and velocities than older technology breakers. When applying modern technology circuit breaker elements, as retrofit or replacement circuit breakers, for the older technology circuit breakers, worn MOC assemblies may possibly create problems.
Older switchgear MOC assemblies were designed for the circuit breaker technology of the time. Older technology circuit breakers employed relatively massive mechanisms, with relatively high forces and inertias. The resultant travel times and velocities were fairly slow such that, for example, the MOC switch may close in approximately 100 ms, measured from the time the circuit breaker begins to close.
The MOC assembly is typically the only component outside the circuit breaker that affects the dynamics of the circuit breaker. The loading and dynamic characteristics of the MOC assembly are highly variable, as the existing MOC assembly typically may have been in service for some time, with mechanical wear and degradation.
The modern technology vacuum circuit breaker is typically much smaller and lighter than a magnetic air circuit breaker. The vacuum breaker has a much smaller travel specification than the magnetic air breaker and operates much faster. When the state of the art vacuum breaker is used with an existing MOC assembly, the resultant speed of operation of the MOC assembly becomes much faster than with a magnetic air breaker. This results in higher impact loads and mismatched inertias with existing MOC assemblies.
For example, with the state of the art vacuum breaker directly driving the MOC assembly, the vacuum breaker and the MOC assembly complete the closing stroke in about 25 ms versus about 100 ms for the magnetic air breaker. The velocity of the MOC assembly is essentially about four times what it was with the magnetic air breaker. Since kinetic energy is proportional to velocity squared, the kinetic energy of the MOC assembly has increased to about 16 times that of the original design. A similar velocity multiplication also occurs during the opening stroke.
This dramatic increase in kinetic energy may typically create the following results: (1) significant MOC switch overtravel and bounce; (2) the MOC switches malfunction during overtravel and bounce; (3) increased MOC assembly component wear; and (4) increased MOC assembly component breakage. In other words, the new vacuum breaker operates too fast for the existing MOC assembly.
Another aspect of this situation is potentially much more serious. Because the MOC assembly is driven directly by the circuit breaker, the MOC assembly affects the circuit breaker dynamics. In the worst case, a badly worn or broken MOC assembly may stall the circuit breaker.
There remains a need, therefore, for an improved MOC operator mechanism for an electrical switching apparatus which slows down the operation of the MOC assembly.
There is also a need for an improved MOC operator mechanism for an electrical switching apparatus which precludes the stalling of such apparatus by the MOC assembly.
There is a more particular need for such a MOC operator mechanism which increases the reliability of the electrical switching apparatus and the MOC assembly.
There is another more particular need for such a MOC operator mechanism which permits the user to evaluate the condition of the MOC assembly.